1. Field of the Invention
The present invention relates to an image processing apparatus that carries out halftoning of multi-tone image data and a printing apparatus that creates dots according to halftone data obtained as a result of the halftoning process and thereby prints an image.
2. Description of the Related Art
Ink jet printers have widely been used as the output device of images processed by the computer. The ink jet printer creates dots on a printing medium with ink ejected from a plurality of nozzles formed on a print head, so as to record an image. The ink jet printer is generally capable of expressing only two tones, that is, the dot-on state and the dot-off state, with regard to each pixel. Image processing generally called the halftoning process is accordingly required, prior to printing an image. The halftoning process enables the multiple tones of original image data to be expressed by a distribution of dots. The ink jet printer is able to print images processed by the computer without the plate making process and thus has great facility.
In the ink jet printer, some effort is made to lower the visual conspicuousness of dots, in order to improve the granularity of the resulting image. The halftoning process adopted in the ink jet printer thus prevents local concentration of dots and attains the good dispersibility of dots. The dithering method using a discrete dither matrix like a Beiyer matrix is the known method to attain such halftoning process. The dithering method determines the dot on-off state, based on comparison between the tone values of the respective pixels and threshold values stored in a preset dither matrix.
The screen printing technique that uses a plate provided for each color is generally applied to print a large quantity of images. In the screen printing, the halftoning process is used for tone expression. The halftoning technique varies the dot percent to express each tone value. FIG. 17 shows an example of tone expression by the halftoning technique. In this example, the tone level is varied in three stages. The upper-most figure corresponds to the lowest tone level, and the tone level is heightened towards the bottom. In the area of low tone, dots having a small dot percent are used for printing. With an increase in tone value and an increase in density to be expressed, dots having the greater dot percent are used for printing.
The screen printing is a suitable technique for mass printing but takes a high cost for plate making. With a view to reducing the cost required for plate making, the ink jet printer may be used for prepress. The prepress here means the trial printing before the actual plate making to allow the operator to check the printed image. The prepress using the ink jet printer advantageously reduces the cost required for plate making.
When the ink jet printer is used for prepress in the screen printing, it is desirable that the picture quality of the printed image by the ink jet printer is close to the picture quality in the screen printing. For this purpose, the halftoning process for the prepress adopts the dithering method using a halftone dot-simulating dither matrix (hereinafter referred to as the halftoning dither). As described previously, the halftoning process in the ink jet printer is generally carried out to ensure the sufficient dispersibility of dots. The dither matrix used in the dot distributed-type halftoning process is set to make the pixels with the high probability of dot creation, that is, the pixels having lower threshold values, appear in a discrete manner in the matrix. In the halftoning dither method, on the other hand, the dither matrix used for the halftoning process is set to make the pixels having lower threshold values locally concentrated and thereby create dots according to a halftone-dot simulating pattern.
The halftoning process using the halftoning dither may, however, extremely lower the picture quality of resulting images due to various factors discussed below. FIG. 18A and FIG. 18B show the state of dots formed by an ink jet printer. FIG. 18A shows the state of dots created in the case of applying the halftoning dither, and FIG. 18B shows the state of dots created in the case of applying the discrete dither. In this example, a large matrix used corresponds to an area CE including a total of 100 pixels (10 pixels in length by 10 pixels in width). The tone level is varied in three stages, where the left part of the area CE expresses image data of high tones and the right part expresses image data of low tones. Each small matrix represents a pixel, and each closed circle represents a dot. The discrete dither gives dots in a distributed manner, whereas the halftoning dither gives local concentration of dots. I the case of the halftoning dither, the resolution is lowered by the local concentration of dots.
The reduced area CE and the shortened pitch of halftone dots can prevent the decrease in resolution due to the above reasons. This arrangement, however, narrows the expressible tone range. When the area CE includes 100 pixels as in the case of FIG. 18A and FIG. 18B, the dot percent may be varied in 100 stages at most. The reduced area CE decreases the number of stages of varying the dot percent, thus narrowing the expressible tone range. The halftoning dither method thus can not attain printing of the appropriate picture quality as the prepress with the sufficient resolution and expressible tone range.
Another problem of the halftoning dither method is the possible deterioration of the picture quality due to the interference of the frequency of halftone dot creation with various frequencies intrinsic to the ink jet printer, for example, the frequency of main scan and the frequency of sub-scan. The ink jet printer generally has a large number of nozzles for ink ejection, and there is a variation in ink ejection characteristics among the respective nozzles. There may be a feeding error in the course of sub-scan. In the ink jet printer, the misalignment of dots due to the varying ink ejection characteristics and the feeding error in the course of sub-scan periodically varies in an image.
The halftoning dither technique gives local concentration of dots as shown in FIGS. 17 and 18. The local concentration of dots appear at the frequency corresponding to the magnitude of the halftone area CE. In some cases, the frequency of causing the local concentration of dots is close to the frequency of significant misalignment of dots. The closed frequencies emphasize the misalignment of dots and cause periodical unevenness of density, thus deteriorating the picture quality of the resulting image.
Each dot blots and expands over the pixel interval on the printing medium. In the distributed dot creation technique, the dot recording rate that is significantly lower than 100% causes the whole surface of the printing medium to be covered with ink in a substantially homogeneous manner. The halftoning dither technique, on the other hand, forms local concentration of dots. Regardless of some blotting, it is difficult to completely cover the whole surface of the printing medium with ink at the dot recording rate that is not very close to 100%. As in the example of the high tone area shown in FIG. 17, the halftoning dither technique may result in dropping out. The printing medium generally has a restriction in absorbable quantity of ink (hereinafter referred to as the duty restriction). Some printing media in specific print modes reach the duty restriction even at the dot recording rate that is significant lower than 100%. Especially in the case of recording a plurality of different color inks in an overlapping manner, there is a severe restriction in quantity of ink for each color. In such cases, the prior art halftoning dither method leaves some undesirable drop-outs to deteriorate the picture quality and moreover does not attain the sufficient density.
Because of the problems discussed above, the halftoning dither does not attain the picture quality in the screen printing, and is thus not suitably applicable for prepress. These problems are found not only in the ink jet printers but in other printing apparatuses that print an image with dots. The essential part of these problems is ascribed to the halftoning process using the halftoning dither, and is thus common to any halftoning processes that create dots according to a diversity of patterns other than the halftone dot-simulating pattern.
The object of the present invention is thus to provide a technique that carries out the halftoning process using a predetermined pattern and improves the picture quality of resulting image based on the results of the halftoning process.
At least part of the above and the other related objects is attained by an image processing apparatus that generates halftone data, which specifies a dot creating state in each pixel, from image data having tone values in a predetermined range. The image processing apparatus includes: an input unit that inputs the image data; and a halftoning unit that carries out a dot distributed-type halftoning process to generate the halftone data. The halftoning unit has: a pattern storage unit that stores noise data according to a predetermined pattern, which includes local concentration of dots; and a reflection unit that causes the noise data stored in the predetermined pattern to be reflected on the halftoning process.
The reflection of noise data here means that at least part of the data used in the course of the halftoning process is corrected with the noise data. One applicable procedure adds noise data to the processed data. The object of the reflection of noise data may be selected according to the requirements.
For example, the noise data may be reflected on the image data.
When the halftoning unit carries out the halftoning process, based on comparison between the image data and predetermined threshold value data, the reflection unit causes the noise data to be reflected on the threshold value data.
The image processing apparatus of the present invention uses the noise data including the local concentration of dots in combination with the dot distributed-type halftoning process. Reflection of the noise data according to the predetermined pattern varies the probability of dot creation, thus attaining the expression according to the predetermined pattern. The dot distributed-type halftoning process increases dots with an increase in tone value while ensuring a sufficient dispersibility. The dispersibility of dots is one factor of improving the picture quality in the expression of an image with dots. The image processing apparatus of the above arrangement ensures the sufficient dispersibility of dots in the expression according to the predetermined pattern and thereby enhances the picture quality of the resulting image by the halftoning process.
The functions of the present invention are discussed below, based on the comparison between the prior art halftoning dither technique and the arrangement of the present invention that applies halftone noise data for the predetermined pattern and adds the noise data to the image data. The halftoning dither technique carries out halftoning of image data using a halftone dot-simulating dither matrix as described previously. This prior art technique performs the halftoning without any correction of the image data itself. This corresponds to the halftoning process for tone expression with the dot percent with regard to the area CE shown in FIGS. 17 and 18 set as the pixel unit. This halftoning process may result in deterioration of the resolution and the expressible tone range and cause problems like occurrence of the unevenness of density due to the periodical appearance of concentrated dots.
In the arrangement of the present invention, addition of the noise data to image data is equivalent to varying the image data itself according to the halftone dot-simulating pattern. This is not the simple dot, but a halftone dot with varying density, where the density gradually decreases from the center of the halftone dot toward the periphery. The technique of the present invention provides the image data following the halftone dot-simulating pattern of such characteristics and then carries out the dot distributed-type halftoning process. The combination of the dot distributed-type halftoning process with the image data following the halftone dot-simulating pattern significantly relieves the problems found in the prior art halftoning dither method and attains a high-quality image simulating halftone dots.
In the above description, the noise data are added to the image data. In the dot distributed-type halftoning process that determines the dot creating state based on the comparison between the image data and the predetermined threshold value, reflection of noise data on the image data is relatively equivalent to reflection of the noise data having the inverted sign on the threshold value. In the technique of the present invention, the noise data may thus be reflected on the threshold value. Another possible procedure makes the noise data reflected on both the image data and the threshold value.
The error diffusion method and the dithering method are applicable techniques to determine the dot creating state in each pixel based on comparison between the image data and a predetermined threshold value. The present invention may use either one of these techniques. The halftoning process here includes the binarization process that determines the dot on-off state in each pixel and the multi-valuing process that determines the dot creating state expressible by three or more different values in each pixel.
The error diffusion method diffuses a quantization error made by each processed pixel to peripheral unprocessed pixels with predetermined weights, makes the total of diffused error divisions in a pixel of interest reflected on image data with regard to the pixel of interest, and determines the dot creating state in the pixel of interest based on comparison between the processed image data and a predetermined threshold value. The quantization error made by the pixel of interest according to the result of the determination of the dot creating state is further diffused to peripheral unprocessed pixels. The error diffusion method minimizes the mean local quantization error and carries out the halftoning process to ensure the excellent picture quality of the resulting image and the sufficient dispersibility of dots. The image processing apparatus that adopts the error diffusion method can thus implement the halftoning process to ensure the high picture quality of the resulting image. Reflection of noise data on the image data in the error diffusion method causes the noise data to be included in the calculation and diffusion of errors. Reflection of noise data on the threshold value, on the other hand, causes the noise data to have no effects on the calculation and diffusion of errors. Either of the procedures of reflection may be adopted in the arrangement of the present invention. The former procedure of reflection causes the effects of the noise data to be relatively remarkable.
The dithering method determines the dot creating state in each pixel based on the comparison between the image data and a threshold value stored in a preset dither matrix. The dither matrix used here may be any of those ensuring the sufficient dispersibility of dots, for example, the Beiyer type. In such a dither matrix, consecutive threshold values appear in a discrete manner. The image processing apparatus that adopts the dithering method can implement the halftoning process at a high speed. In the dithering method, noise data may be reflected on either the image data or the threshold value. These two procedures of reflection give the equivalent results in the case of the dithering method.
The halftoning process by the dithering method may use a specific matrix obtained by previously adding noises of a predetermined pattern to a dither matrix ensuring the sufficient dispersibility of dots. This procedure corresponds to one application of the reflection of the noise data on the threshold value. This arrangement omits the step of reflecting the noise data on either the image data or the threshold value in the halftoning process and thus enhances the processing speed.
The above example applies the halftone noise data for the predetermined pattern. The predetermined pattern is, however, not restricted to the pattern of halftone dots. The present invention may apply any pattern that includes local concentration of dots, in order to improve the picture quality of the resulting image by the halftoning process. The halftone dot-simulating pattern is only one of the various applicable patterns. The halftone dot-simulating pattern is, however, highly effective since it is applicable for the prepress in the screen printing that utilizes the apparatus of printing or displaying images with dots. The variety of other patterns may be utilized as special effects.
The image processing apparatus of the present invention has other advantages discussed below. In the image processing apparatus of the present invention, the noises of the predetermined pattern have substantially no effects on the technique of the halftoning process. The effects of addition of the predetermined pattern can thus be emphasized or relieved in a flexible manner by simply adjusting the magnitude of the noise data.
A variety of advantages discussed below are expected in the case where the halftone data generated by the image processing apparatus of the present invention are printed with an ink jet printer. Creation of dots within the duty restriction, which depends upon the printing medium, is required to ensure the sufficient picture quality of the resulting image by the ink jet printer. The image processing apparatus of the present invention applies the dot distributed-type halftoning process and thus prevents extreme concentration of dots. The dot distributed-type halftoning process is equivalent to the halftoning process generally used for the printing operations in the ink jet printer. A variety of known techniques are thereby applicable to record dots within the duty restriction. The image processing apparatus of the present invention thus advantageously keep the duty restriction in the application for the ink jet printer.
As described previously as the drawback of the prior art technique, in the ink jet printer, the interference of the frequency of local concentration of dots with the frequency of misalignment of dots created by the respective nozzles causes periodical unevenness of density. The unevenness of density is especially conspicuous in the halftoning dither. The image processing apparatus of the present invention, on the other hand, adopts the dot distributed-type halftoning process and ensures the sufficient dispersibility of dots, thus reducing the unevenness of density due to the interference. The ink jet printer that receives the halftone data processed by the image processing apparatus of the present invention can attain the high quality printing.
In the image processing apparatus of the present invention, the image data may be monochromatic data or color data.
When the image data is multi-color image data, the reflection unit causes the predetermined pattern, which is a halftone dot-simulating pattern having different screen angles between at least part of colors, to be reflected on the halftoning process.
In this case, it is preferable that the at least part of colors are cyan and magenta.
As described previously, the interference of the frequency of local concentration of dots with the frequency of misalignment of dots created by the printing apparatus deteriorates the picture quality of the resulting image. Application of the pattern having an identical screen angle for all the colors results in the significant appearance of such interferences. The preferable arrangement of the present invention discussed above applies the predetermined pattern having different screen angles between at least part of colors, in order to relieve the potential effects due to the interference. As is known in the general screen printing, formation of halftone dots with different screen angles adopted for the respective colors enables the tone expression closer to that in the screen printing. Different screen angles may be set for all the colors, or alternatively a common screen angle may be set for part of the colors. Setting the different screen angles for cyan and magenta, which have relatively high visual recognizability, effectively reduces the interference and enables the tone expression similar to that in the screen printing.
In accordance with one preferable application of the image processing apparatus of the present invention, when the dots include a plurality of different types of dots having at least one of different hues and different reflective densities of creation, the halftoning unit carries out the halftoning process for each type of dots and causes the noise data to be reflected on the halftoning process with regard to at least part of the dots.
The dots having different reflective densities represent dots created by inks of different densities or dots having different quantities of ink.
The noise data include local concentration of dots. Some degree of dispersibility of dots is desirable to improve the picture quality in the apparatus of printing or displaying images with dots. The arrangement of reflecting the noise data with regard to the part of the dots ensures the sufficient dispersibility of the dots without reflection of the noise data, thus enhancing the total picture quality of the resulting image. Reducing the number of the types of dots created in a locally concentrated manner makes easier to keep the duty restriction.
Reducing the number of the types of dots with addition of the noise pattern effectively prevents the deterioration of the picture quality due to the interference of the frequency of local concentration of dots with the various frequencies intrinsic to the printing apparatus. This arrangement further relieves the labor of adding the noise pattern and reduces the number of the noise patterns to be provided.
In the arrangement of causing the noise data to be reflected on the part of the dots, when the dots include a plurality of different types of dots having an identical hue but different reflective densities of creation, the part of the dots excludes at least a specific type of dots having the identical hue and a higher reflective density among the plurality of different types of dots.
For example, in the case of dots having different quantities of ink, the noise data may be reflected on the dots having the smaller quantity of ink. In the case of dots created with inks of different densities, the noise data may be reflected on the dots created with ink of the lower density. The similar technique is adopted in the case of creating a plurality of different types of dots by combining different quantities of ink and different densities of inks.
It is not necessary that the above condition is fulfilled for all the hues. For example, in the case where the two colors, cyan and magenta, respectively provide a plurality of different dots having different reflective densities, the noise data may be reflected on only the dots having the lower reflective density with regard to both the colors or with regard to only one of the colors.
Addition of the noise data to the dots having the higher reflective density results in forming the predetermined pattern in a high density area. The pattern formed in such a high density area is visually rather inconspicuous, so that the addition of the predetermined pattern does not have significant effects on the tone expression. Addition of the noise data to the other dots, that is, the dots used in a low tone area or an intermediate tone area, on the other hand, forms the predetermined pattern that is visually conspicuous. This ensures the significant effects.
In the arrangement of causing the noise data to be reflected on the part of the dots, the noise data may be reflected on the dots having hues excluding at least yellow. For example, the dots, which are the object of reflection, may have the hues of cyan and magenta. Black dots may also be the object of reflection of the noise data. The color of yellow is visually rather inconspicuous, and no significant effects are expected by addition of the noise data to the yellow dots. The arrangement of omitting the reflection on the yellow dots, on the other hand, advantageously shortens the total time required for the processing and reduces the possible interference.
The principle of the present invention may be attained by a printing apparatus having a main part identical with that of the image processing apparatus discussed above.
The present invention is accordingly directed to a printing apparatus that creates dots to print an image on a printing medium. The printing apparatus includes: an input unit that inputs image data having tone values in a predetermined range; a halftoning unit that carries out a dot distributed-type halftoning process to generate halftone data, which specifies a dot creating state in each pixel; and a dot creation unit that creates dots on the printing medium based on the halftone data. The halftoning unit has: a pattern storage unit that stores noise data according to a predetermined pattern, which includes local concentration of dots; and a reflection unit that causes the noise data stored in the predetermined pattern to be reflected on the halftoning process.
The printing apparatus of the present invention prints an image according to the halftone data generated by the series of the processing, which is similar to the processing discussed previously with regard to the image processing apparatus. This arrangement ensures the high-quality printing with the additional effects of the predetermined pattern on the tone expression. The variety of additional factors described previously with regard to the image processing apparatus are also applicable to this printing apparatus. Any of various printing apparatuses that print an image with dots may be applied for the printing apparatus of the present invention. The ink ejection-type printing apparatus that ejects ink to create dots is especially suitable for the technique of ensuring the sufficient dispersibility of dots and thereby attaining the high-quality printing.
The present invention is further directed to an image processing method that generates halftone data, which specifies a dot creating state in each pixel, from image data having tone values in a predetermined range. The method includes the steps of: (a) inputting the image data; and (b) carrying out a dot distributed-type halftoning process, on which noise data preset according to a predetermined pattern including local concentration of dots are reflected, so as to generate the halftone data.
Because of the same effects as those discussed previously in the image processing apparatus, the image processing method of the present invention improves the picture quality of a resulting image that includes dots created according to a predetermined pattern and is subjected to the halftoning process. The variety of additional factors described previously with regard to the image processing apparatus are also applicable to this image processing method. The present invention is attained by a printing method, as well as the image processing method.
The present invention is also directed to a recording medium in which a specific program is recorded in a computer readable manner, wherein the specific program generates halftone data, which specifies a dot creating state in each pixel, from image data having tone values in a predetermined range. The specific program includes noise data preset according to a predetermined pattern including local concentration of dots. The specific program causes a computer to attain the functions of: causing the noise data to be reflected at least either one of input image data and a predetermined threshold value; and carrying out a dot distributed-type halftoning process to generate the halftone data from the image data and the threshold value after the reflection.
The computer executes the specific program recorded in the recording medium, so as to implement the image processing of the present invention. The specific program may be constructed as an individual program for attaining the above functions or alternatively as part of the program for driving the printing apparatus.
Typical examples of the recording medium include flexible disks, CD-ROMs, magneto-optic discs, IC cards, ROM cartridges, punched cards, prints with barcodes or other codes printed thereon, internal storage devices (memories like a RAM and a ROM) and external storage devices of the computer, and a variety of other computer readable media. The principle of the present invention may also be attained by a program supply unit, from which the computer program is supplied to the computer, as well as the computer program itself and a diversity of equivalent signals.
These and other objects, features, aspects, and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with the accompanying drawings.